1. Field of the Invention
The present invention relates to a method and apparatus for inputting letters, and more particularly, to a method and apparatus for inputting letters via a simple manipulation by using a much less number of keys than the total number of alphabet letters.
2. Description of the Related Art
As well known, English is one of official international languages used in international conferences, etc., and is being widely used as a second language in many countries. In English, two or more of twenty six letters are combined to represent one word having a particular meaning.
On a QWERTY keyboard commonly used for computers or the like, twenty six letters are respectively assigned to different keys so that a user may easily input a desired word or sentence by pressing keys corresponding to the appropriate letters.
Recently, due to the rapid development of electronic technologies, various compact and portable devices such as cellular phones including smart phones, Moving Picture Experts Group (MPEG) audio layer 3 (MP3) players, digital multimedia broadcasting (DMB) terminals, satellite terminals, personal digital assistants (PDAs), and portable multimedia players (PMPs) have become very popular. In this regard, as the size of such devices can be the most competitive marketing advantage, manufactures have made continuous efforts to produce thin, light, and compact portable devices.
As such, portable devices typically have a keypad that contains a limited number of keys, for example, twelve letter keys, and a few additional function keys.
FIG. 1 illustrates a conventional phone-key keypad for inputting numbers and English letters.
This type of keypad is common in coded phones, cellular phones, etc., and is refereed to as a phone-key keypad. In the phone-key keypad, from among twenty six letters, letters Q and Z are assigned to key number 1, letters A, B, and C are assigned to key number 2, letters D, E, and F are assigned to key number 3, letters G, H, and I are assigned to key number 4, letters J, K, and L are assigned to key number 5, letters M, N, and 0 are assigned to key number 6, letter P, R, and S are assigned to key number 7, letters T, U, and V are assigned to key number 8, and letters W, X, and Y are assigned to key number 9.
In the phone-key keypad having the above arrangement, a letter is input by pressing a specific key a certain number of times. For example, if a user desires to input the text “call me john”, the keypad has to be manipulated as follows: triple pressing of key number 2→(a pause or a space)→single pressing of key number 2→triple pressing of key number 5→(a pause or a space)→triple pressing of key number 5→(a pause)→(a space)→single pressing of key number 6→double pressing of key number 3→(a pause or a space)→a space→single pressing of key number 5→triple pressing of key number 6→double pressing of key number 4→double pressing of key number 6.
In this case, since there is no correlation between the letters assigned to each key and no rule or logic with respect to the number of times that a key is pressed to input a specific letter, a user needs to be very careful when inputting desired letters. Also, an error in pressing a key frequently occurs because a plurality of letters are assigned to one key, and a large amount of time or a larger number of key pressings is required because a pause or pressing a space key is required. In order to solve this problem, Korean Patent Registration No. 10-0672773 entitled Alphabet Input Apparatus and Alphabet Input Method and registered on Dec. 27, 2006 (hereinafter referred to as cited reference 1) discloses a method of separating and symbolizing strokes of letters to symbolize the strokes and combining the symbols with each other to input a desired letter.
In cited reference 1, symbols such as /, |, \, , —, , V, O, and  are assigned to nine keys and one to four of the symbols are combined to input a desired letter. The letters are expressed by selecting the symbols as described below.
A: sequential pressing of /, \, and —
B: sequential pressing of |, , and ⊂
C: single pressing of ⊂
D: sequential pressing of | and ⊃
E: sequential pressing of |, —, —, and —
F: sequential pressing of |, −, and —
G: sequential pressing of , —, and |
H: sequential pressing of |, —, and |
I: single pressing of |
J: sequential pressing of | and 
K: sequential pressing of |, /, and \
L: sequential pressing of | and —
M: sequential pressing of |, \, /, and |
N: sequential pressing of |, \, and |
O: single pressing of O
P: sequential pressing of | and ⊃
Q: sequential pressing of O and \
R: sequential pressing of |, , and \
S: sequential pressing of  and ⊃
T: sequential pressing of — and |
U: single pressing of ∪
V: single pressing of V
W: sequential pressing of V and V
X: sequential pressing of \ and /
Y: sequential pressing of V and |
Z: sequential pressing of —, /, and —
Although the apparatus and method disclosed in cited reference 1 are convenient to input a letter by combining strokes of letters, users still have difficulties in using them because some letters require pressing a key four times and the above keypad assignment is not as instinctively and easily recognizable as handwriting. In cited reference 1, a total of sixty key pressings are required to input all alphabet letters.